ReferenceError: process is not defined, Error in h

问题:

Just deployed an app (https://socket-draw-app.herokuapp.com/) on Heroku but am getting this error: ReferenceError: process is not defined. in a javascript file: sketch.js. I am using the p5.js library. Code below (firstly the sketch.js and then my server.js node file) :

var socket;

function setup() {
  createCanvas(400, 400);
  background(0);
  socket = io.connect(process.env.PORT || 3000);
  socket.on('mouse',
    
    function(data) {
      console.log("Got: " + data.x + " " + data.y);
      
      fill(0,0,255);
      noStroke();
      ellipse(data.x, data.y, 20, 20);
    }
  );
}

function draw() {
  // Nothing
}

function mouseDragged() {
  // Draw some white circles
  fill(255);
  noStroke();
  ellipse(mouseX,mouseY,20,20);
  // Send the mouse coordinates
  sendmouse(mouseX,mouseY);
}

// Function for sending to the socket
function sendmouse(xpos, ypos) {
  // We are sending!
  console.log("sendmouse: " + xpos + " " + ypos);
  
  // Make a little object with  and y
  var data = {
    x: xpos,
    y: ypos
  };

  // Send that object to the socket
  socket.emit('mouse',data);
}

var express = require('express');
// Create the app
var app = express();

// Set up the server
// process.env.PORT is related to deploying on heroku
var server = app.listen(process.env.PORT || 3000, listen);

// This call back just tells us that the server has started
function listen() {
  var host = server.address().address;
  var port = server.address().port;
  console.log('Example app listening at http://' + host + ':' + port);
}

app.use(express.static('public'));


// WebSocket Portion
// WebSockets work with the HTTP server
var io = require('socket.io')(server);

// Register a callback function to run when we have an individual connection
// This is run for each individual user that connects
io.sockets.on('connection',
  // We are given a websocket object in our function
  function (socket) {
  
    console.log("We have a new client: " + socket.id);
  
    // When this user emits, client side: socket.emit('otherevent',some data);
    socket.on('mouse',
      function(data) {
        // Data comes in as whatever was sent, including objects
        console.log("Received: 'mouse' " + data.x + " " + data.y);
      
        // Send it to all other clients
        socket.broadcast.emit('mouse', data);
        
        // This is a way to send to everyone including sender
        // io.sockets.emit('message', "this goes to everyone");

      }
    );
    
    socket.on('disconnect', function() {
      console.log("Client has disconnected");
    });
  }
);

{
  "name": "sockets_p5",
  "version": "1.0.0",
  "description": "This is an example for how to use sockets with p5.js.",
  "main": "server.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "Daniel Shiffman",
  "license": "ISC",
  "dependencies": {
    "express": "^4.16.2",
    "socket.io": "^1.4.5"
  }
}

Was deployed from my machine running Ubuntu 16.04. Node version 4.2.6 and Npm version 3.5.2 .